FIG. 1 shows a conventional power supply circuit for a brushless direct current motor M. The power supply circuit includes a full bridge circuit having a first switching circuit 11 and a second switching circuit 12. The motor is arranged in the transverse branch of the full bridge circuit. Four switches K1˜K4 of the bridge circuit are controlled to be conductive or blocked by four signals SN1˜SN4. The term blocked refers to the switch being in an open or non-conductive state.
When the switches K1, K4 are conductive and the switches K2, K3 are blocked, current passes through the motor in a direction from a node a1 to a node a2. When the switches K2, K3 are conductive and the switches K1, K4 are blocked, current passes through the motor in a direction from the node a2 to the node a1. By controlling the two pairs of switches K1, K4 and K2, K3 to be alternately conductive, alternating current is supplied to the motor, which drives the motor to continuously rotate in a certain direction.
For some different applications driven by motors, such as two different fans whose curved vanes bends toward opposite directions, the motors are required to rotate in both directions. It is desired to apply a same power supply circuit to the different applications with minimal modifications.